Chemical Imaging of Carbide Formation and Its Effect on Alcohol Selectivity in Fischer Tropsch Synthesis on Mn-Doped Co/TiO2 Pellets

X-ray diffraction/scattering computed tomography (XRS-CT) was used to create two-dimensional images, with 20 μm resolution, of passivated Co/TiO2/Mn Fischer–Tropsch catalyst extrudates postreaction after 300 h on stream under industrially relevant conditions. This combination of scattering techniques provided insights into both the spatial variation of the different cobalt phases and the influence that increasing Mn loading has on this. It also demonstrated the presence of a wax coating throughout the extrudate and its capacity to preserve the Co/Mn species in their state in the reactor. Correlating these findings with catalytic performance highlights the crucial phases and active sites within Fischer–Tropsch catalysts required for understanding the tunability of the product distribution between saturated hydrocarbons or oxygenate and olefin products. In particular, a Mn loading of 3 wt % led to an optimum equilibrium between the amount of hexagonal close-packed Co and Co2C phases resulting in maximum oxygenate selectivity. XRS-CT revealed Co2C to be located on the extrudates’ periphery, while metallic Co phases were more prevalent toward the center, possibly due to a lower [CO] ratio there. Reduction at 450 °C of a 10 wt % Mn sample resulted in MnTiO3 formation, which inhibited carbide formation and alcohol selectivity. It is suggested that small MnO particles promote Co carburization by decreasing the CO dissociation barrier, and the Co2C phase promotes CO nondissociative adsorption leading to increased oxygenate selectivity. This study highlights the influence of Mn on the catalyst structure and function and the importance of studying catalysts under industrially relevant reaction times.

Figure S12 -XRD NMF-generated masks highlighting the spatial intensity variations of the 5 components within the catalytic samples.Component 1 exhibited similarities to the FCC/HCP phases, which tended to concentrate in the core of the catalyst at higher Mn loadings, while Component 3 contained the Co2C phase which was found on the catalyst periphery.
Figure S1 -Experimental set-up at ID31, ESRF, for the XRD-CT and PDF-CT experiments

Figure S3 -
Figure S3 -CO conversion and selectivity to C5+, alcohols and olefins for the 0 % and 10 % Mn (450 °C) sample for the reaction time from 160 to 300 h at 30 barg and 210-240 °C.

Figure S4 -
Figure S4 -CO conversion and selectivity to C5+, alcohols and olefins for the 0% and 3% Mn sample for the reaction time from 160 to 300 h at 30 barg and 210-240 °C.

Figure S5 -
Figure S5 -CO conversion for all the samples tested from 160 -270 h at 30 barg and 210-240 °C.

Figure S6 -
Figure S6 -XRD pattern of the CeO2 calibrant, the calculated refined pattern and the difference pattern.

Figure S7 -
Figure S7 -Full mean XRD pattern of the samples with different Mn loading with indexing lines of all the phases present.FCC and HCP Co phases were present at low Mn loading (0-2 % Mn) and Co2C was present at higher Mn loading (3-10 % Mn).A low percentage of Cox Mn1-x O was present at higher Mn loadings (5-10 %) but its peaks were underneath the Co2C peaks.Support phases, anatase and rutile, were present in all samples and MnTiO3 was present in the sample reduced at 450 °C.

Figure S8 -
Figure S8 -Mean pattern refinement of the 3 % Mn reacted sample highlighting the 2 ° region with reflections from the different Co containing phases (Co2C, FCC and HCP cobalt metal phases respectively).The experimental, refined and difference XRD patterns are displayed as well as the indexing lines for the reference patterns below.The peak shapes are not fitted exactly due to the presence of stacking faults in the FCC/HCP phase, but the Rwp is still low at 6.2 %.

Figure
Figure S9 -XRD patterns of the 0-3 % Mn samples from the centre of the extrudates after 300 h of reaction.The 3 % Mn sample has more prominent HCP peaks at 3.60° and 4.07°.

Figure S10 -
Figure S10 -Rwp maps for the XRD-CT and PDF-CT refinements where the XRD-CT Rwp was between 8 -10 % indicating a good fit for all the samples.Increasing Mn loading resulted in a larger Rwp indicating increased disorder.The PDF-CT Rwp was between 23 -28 % which indicates a tolerable fit for PDF data.

Figure S11 -
Figure S11 -Reconstructed XRD-CT 2D images of the catalytic pellets (extracted after 300 h) at alternative heights illustrating the refined wt.% percentage (left) and crystallite size (right) of the different phases present.The results at different heights show the same increase in Co2C with Mn loading and egg-shell distribution with FCC/HCP in the centre and carbide on the periphery of the pellets.

Figure S13 -
Figure S13 -XRD patterns of the NMF-generated components.All components contained the support phases as the support was homogeneous.Component 1 contained predominantly FCC/HCP Co, component 3 contained Co2C and component 2 contained MnTiO3.Components 4 and 5 were multiplied by 5 and 100, respectively, for visibility.

Figure
Figure S14 -XRD-CT maps of the refined parameters of the wax content (wt.%), lattice parameters (LPA, LPB and LPC), and crystallite size (CS).*Reduced at 450 °C during preparation.

Figure S16 -
Figure S16 -PDF of the NMF-generated components.All components contained the support phases as the support was homogeneous.Component 1 contained predominantly FCC/HCP Co, component 5 contained Co2C and component 3 contained MnTiO3.

Figure
Figure S17 -µ-XRF images (intensity (%)) of the cross-sections (right) of the 3 wt.% Mn samples extracted before and afterreaction for 300 h.The Mn and Co are found to be co-located.

Figure
Figure S18 -µ-XRF images (intensity (%)) of the lateral faces of the 3 wt.% Mn samples extracted before and after reactionfor 300 h.

Table S1 -
Crystallographic information of identified phases.

Table S2 -
Rietveld Refinement results of the XRD-CT mean patterns illustrating the phase wt.%, CS (crystallite size in nm) and LP (lattice parameters) of the samples extracted after 150 h and 300 h.The Co2C wt.% increases with Mn loading whilstthe Co 0 metal phases (FCC and HCP) decrease with increasing Mn wt.% beyond 5 %.The 10 % samples reduced at 450 °C resulted in the production of Mn titanates (MnTiO3).aReduction at 450 °C a Reduction at 450 °C

Table S4 -
XRD Rietveld refinement results from refining the NMF-generated components.Component 1 was found to contain FCC/HCP phases whilst component 2 contained the MnTiO3.Component 3 contained the Co2C phase whilst components 4 and 5 had a mixture of different phases.% int.refers to the percentage intensity of each component.

Table S5 -
Errors from the XRD Rietveld refinement of the NMF-generated components.

Table S6 -
Real-space Refinement results of the PDF-CT mean patterns illustrating the phase wt.%, CS (crystallite size) and LP (lattice parameters).The results correlate well with the XRD refinement.Co2C wt.% increases with Mn loading whilst theCo 0 metal phases (FCC and HCP) decrease with increasing Mn wt.% beyond 5 %.
TableS7-Errors from the Real-space Refinement results of the PDF-CT mean patterns for the phase wt.%, CS (crystallite size) and LP (lattice parameters).aReduction at 450° C